Linear thermoplastic polyesters such as poly(alkylene terephthalate) are generally known and commercially available where the alkylene typically has 2 to 8 carbon atoms. Linear polyesters have many valuable characteristics including strength, toughness, high gloss and solvent resistance. Linear polyesters are conventionally prepared by the reaction of a diol with a dicarboxylic acid or its functional derivative, typically a diacid halide or diester. Linear polyesters may be fabricated into articles of manufacture by a number of known techniques including extrusion, compression molding and injection molding.
Recently, macrocyclic polyesters oligomers were developed which have unique properties that make them attractive as matrices for engineering thermoplastic composites. The desirable properties stem from the fact that macrocyclic polyester oligomers exhibit low melt viscosity, allowing them easily to impregnate a dense fibrous preform followed by polymerization to polyesters. Furthermore, certain macrocyclic polyester oligomers melt and polymerize at temperatures well below the melting point of the resulting polymer. Upon melting and in the presence of an appropriate catalyst, polymerization and crystallization can occur virtually isothermally.
The preparation of macrocyclic poly(alkylene dicarboxylate) oligomers and their polymerization to linear polyesters is described in U.S. Pat. Nos. 5,039,783, 5,214,158, 5,231,161, 5,321,117, and 5,466,744; and is reviewed by D. J. Brunelle in Cyclic Polymers, Second Edition [J. A. Semlyn (ed.), (2000), Kluwer Academic Publishers (Netherlands), pp. 185-228]. The catalysts employed for such polymerization include various organotin compounds and titanate esters, usually in solution polymerization processes. Polymerization using these catalysts is particularly successful in the case of poly(1,4-butylene terephthalate) (“PBT”) because of the low temperatures at which the polymerization can be carried out. However, catalyst performance is limited by sensitivity to impurities present in the macrocyclic polyester oligomers, particularly acidic impurities. Such catalysts also lack adequate thermal stability at the high temperatures required for some polyester polymerizations. This is particularly the case of poly(1,3-propylene terephthalate) (“PPT”).
Kamau et al. (Polymers for Advanced Technologies, 2003, Vol. 14, pp. 492-501) used di-n-butyltin oxide to catalyze the ring-opening polymerization of a mixture of cyclic PPT oligomers at 300° C. under nitrogen for two hours. The linear polymer so produced had a viscosity average molecular weight of only 22,500. Use of a specially purified PPT dimer increased the viscosity average molecular weight only to 30,300. The long time required and low molecular weight, brittle materials produced indicate this is not a commercially viable process.
N-heterocyclic carbenes have been used to catalyze ring-opening polymerization of aliphatic polyesters and lactones. Connor et al. demonstrated the use of 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene to catalyze the ring-opening polymerization of L-lactide, ε-caprolactone, and β-butyrolactone. The polymerizations were carried out in tetrahydrofuran at room temperature in the presence of an alcohol initiator (J. Am. Chem. Soc., 2002, Vol. 124, pp. 914-915).
There thus remains a need for an effective and efficient high-temperature process for preparing linear polyesters from macrocyclic polyester oligomers.